Structural studies of epitaxial BaTiO₃ film deposited on MgO-buffered r-plane cut sapphire

The microstructure of epitaxial BaTiO₃ thin films on MgO-buffered r-plane cut sapphire substrates was investigated by means of transmission electron microscopy. The BaTiO₃ (BTO) films exhibit orientation relationships with the MgO (MO) buffer layer and the r-plane cut sapphire (AO) substrates of [1 0 0]_BTO∥[1 0 0]_MO∥[1 1 2̄ 0]_AO and (0 1 0)_BTO∥(0 1 0)_MO∥(1̄ 1 0 4)_AO. Following these relationships, the (0 0 1)_BTO and the (0 0 1)_MO planes make an angle of several degrees with the (1 1̄ 0 2)_AO plane, the surface plane of the substrate. A MgAl₂O₄ spinel reaction layer appears at the interface between the MgO buffer layer and the sapphire substrates for the films deposited at high temperature. In thick films (800 nm) cracks penetrating the whole film thickness are developed by the tensile stress induced by the difference in thermal expansion between the film and the substrate. An anisotropic distribution of planar defects such as {1 1 1} stacking faults and microtwins is observed in the BaTiO₃ film. This anisotropy can be explained by the application of the thermal stress to the BaTiO₃ film with its [0 0 1] axis deviating by a small angle from the normal of the substrate surface.